1. Field of the Invention
The invention relates to active devices for high frequency applications, and more particularly to quasi-planar signal processing structures such as mixers in which the entire circuit is disposed in a hermetically sealed module.
2. Description of the Prior Art
In general, active high frequency devices, especially those designed to operate with carrier signals above 30 GHz, have been built in waveguide cavities in order to minimize losses and to permit incorporation in the structures of necessary filters. But when constructed integrally with other receiver components, the arrangement of the various interconnecting waveguides has presented formidable problems barring achievement of fully acceptable cost, package size, and system bandwidth, as well as of sufficiently low losses.
In U.S. Pat. No. 4,276,655 issued June 30, 1981 to E. H. Kraemer and John C. Rolfs, and assigned to the assignee of the present invention, a cross-bar signal mixer is disclosed that comprises a balanced planar section supported upon a dielectric substrate placed across the carrier and local oscillator waveguides perpendicular to the direction of electromagnetic energy propagation. Beam lead mixer diodes are connected across the feed point of a taper matching section. In this embodiment, the planar mixer included a substrate containing the mixer diodes mounted perpendicular to the direction of propagation of the signal-input frequency.
In a fin-line structure described by P. J. Meier in U.S. Pat. No. 3,825,863, issued July 23, 1974, a mixer comprised of a waveguide with a longitudinal film conductor is supported on a dielectric substrate within the guide, using beam-lead diodes mounted on the substrate.
The effectiveness of the preceding mixer circuits at millimeter wavelengths is impaired by stray circuit capacitance. While the beam lead diode performs adequately at frequencies as high as 100 GHz, the passivation of such diodes agains contaminants and, in particular, sodium ions, is difficult due to the necessity of minimizing stray shunt capacity. Stray capacitance as high as 0.03 pF or greater, while acceptable at Ka band, would greatly increase the conversion loss at higher millimeter frequencies (as, for example, above 60 GHz). The result is to increase conversion loss at 100 GHz from a typical value of 4.5 db at a stray capacitance of 0.015 pF to approximately 8 dB for a stray capacitance of 0.035 pF for a fully passivated beam-lead diode. Passivation with silicon dioxide (SiO.sub.2) has been successfully applied to mixer diodes of the beam-lead type up to 140 GHz, where noise figures of 6.0 dB (double sideband) have been achieved. However, SiO.sub.2 is particularly susceptible to invasion by sodium ions, as would be encountered in a saltwater atmosphere. Further, gallium arsenide (GaAs) as used for interconnections in a beam-lead diode is highly susceptible to invasion by moisture, thereby resulting in degraded performance if operation in a humid atmosphere is required. Effective passivation by a sodium ion resistant insulator such as silicon nitride (Si.sub.3 N.sub.4) results in a substantial increase in stray capacitance and, consequently, unacceptable conversion losses. Thus, the beneficial properties of silicon nitride in providing more resistance to ionic contamination than silicon dioxide cannot be utilized. In consequence, the prior art has attempted to provide hermetic sealing of the susceptible devices.
This has heretofore been accomplished by enclosing the entire diode in a hermetically sealed ceramic case. This leads adversely to a higher stray shunt capacity and additional losses due to the ceramic case, and thus reduces the conversion efficiency of the mixer (i.e., increased conversion loss and noise figure). The use of a sealed module housing the active mixer elements and associated circuit has not heretofore been reported. Beneficially, the use of a sealed module reduces the losses since the sealing windows have a larger surface area and, therefore, a reduced current density and consequent lower losses. In addition, its stray reactance can more readily be tuned-out with low loss by proper choice of the window size and thickness. The approach described will also provide wider bandwidth than circuits which attempt to match out the parasitic reactances of a ceramic cased mixer diode.